Controlling Chirogenic Effects in Porphyrin Based Supramolecular Systems: Theoretical Analysis Versus Experimental Observations.

Electronic circular dichroism (ECD) spectroscopy is a widely employed method for studying chiral analysis, requiring the presence of a chromophore close to a chiral centre. Porphyrinoids are found to be one of the best chromophoric systems serving for this purpose and enabling the application of ECD spectroscopy for chirality determination across diverse classes of organic compounds. Consequently, it is crucial to understand the induction mechanisms of ECD in the porphyrin-based complexes. The present study explores systematically the influence of secondary chromophores, bonded to an achiral zinc porphyrin or to chiral guest molecules, on the B-region of ECD spectra using the time-dependent density functional theory (TD-DFT) calculations. The study analyses the impact of change in both the conformation of achiral porphyrin (host) and change in position and conformation of chiral organic molecule (guest) on the B-band of ECD spectra (energy, intensity, sign of Cotton effect). Finally, conclusions made on model complexes are applied to published experimental data, contributing to a deeper understanding of various factors influencing ECD spectra in chiral systems. In addition, a computer program aimed to help rationalise ECD spectra by visualizing corresponding orbital energies, rotatory strengths, electric and magnetic transition moments, and angles between them, is presented.

Linearization of moment tensor potentials for multicomponent systems with a preliminary assessment for short-range interaction energy in water dimer and trimer.

Moment tensor potentials have been recently proposed as a promising novel method of polynomial expansion for the systematic approximation of molecular potential energy surfaces. However, its current formulation for multicomponent systems has not been fully linearized and requires nonlinear optimization techniques for parameter estimation. We propose an alternative relaxed formulation of the original potential energy function where parameter optimization is expressed as a linear sparse approximation problem. The main difficulty arising in sparse approximation is finding a suitable subset of predictors in highly multi-collinear variable space where the number of variables largely exceeds the size of the training set. To efficiently reduce the number of descriptors to an optimal size and prevent overfitting, we present a simple heuristic that is based on importance ranking of variables and Bayesian information criterion. For the empirical assessment of our approach, we employed published data on short-range components of water two-body and three-body interaction energies that have previously been used for the comparison of various potential energy representations. Numerical experiments suggest that our proposed methodology allows achieving accuracy that is comparable to other popular interpolation and machine learning techniques and requires significantly less time for model training than nonlinearly parameterized formulation.

Formation and trapping of the thermodynamically unfavoured inverted-hemicucurbit[6]uril.

Formation of inverted-cis-cyclohexanohemicucurbit[6]uril (i-cis-cycHC[6]), with up to 33% isolated yield, can be induced at the expense of thermodynamically favoured cis-cycHC[6]. Reaction selectivity is governed by the solution-based template-aided dynamic combinatorial chemistry and continuous precipitation of the formed macrocycles. Different binding affinities of three diastereomeric cycHC[6]s with trifluoroacetic acid is demonstrated.

Linking atmospheric, terrestrial and aquatic environments: Regime shifts in the Estonian climate over the past 50 years.

Climate change in recent decades has been identified as a significant threat to natural environments and human wellbeing. This is because some of the contemporary changes to climate are abrupt and result in persistent changes in the state of natural systems; so called regime shifts (RS). This study aimed to detect and analyse the timing and strength of RS in Estonian climate at the half-century scale (1966-2013). We demonstrate that the extensive winter warming of the Northern Hemisphere in the late 1980s was represented in atmospheric, terrestrial, freshwater and marine systems to an extent not observed before or after the event within the studied time series. In 1989, abiotic variables displayed statistically significant regime shifts in atmospheric, river and marine systems, but not in lake and bog systems. This was followed by regime shifts in the biotic time series of bogs and marine ecosystems in 1990. However, many biotic time series lacked regime shifts, or the shifts were uncoupled from large-scale atmospheric circulation. We suggest that the latter is possibly due to complex and temporally variable interactions between abiotic and biotic elements with ecosystem properties buffering biotic responses to climate change signals, as well as being affected by concurrent anthropogenic impacts on natural environments.

Computational and ion mobility MS study of (all-S)-cyclohexylhemicucurbit[6]uril structure and complexes.

A computational study of (all-S)-cyclohexylhemicucurbit[6]uril and its complexes with anions (Cl(-), Br(-), I(-) and HCOO(-)), the proton (H(+)) and non-dissociated acid (HCl, HBr, HI and HCOOH) guests was performed. The geometries of guest-host complexes were optimized via density functional theory using the BP86 functional, SV(P) basis set and Stuttgart pseudopotentials for iodide. Binding affinities and their trends were evaluated at the BP86/TZVPD level of theory. In addition, the quantum theory of atoms in molecules was used to gain insight into guest-host interactions. A computational study in the gas phase and ion-mobility mass-spectrometry analysis revealed that the studied macrocycle formed inclusion complexes with anions. Protonation of the macrocycle is preferred at the nitrogen atom pointing inside of the cavity. In the studied conditions, non-dissociated acids formed complexes at the oxygen atom pointing outside of the macrocycle.

Organocatalytic asymmetric addition of malonates to unsaturated 1,4-diketones.

The organocatalytic Michael addition of malonates to symmetric unsaturated 1,4-diketones catalyzed by thiourea and squaramide derivatives with Cinchona alkaloids afforded the formation of a new C-C bond in high yields (up to 98%) and enantiomeric purities (up to 93%). The absolute configuration of the product was suggested from comparison of the experimental and calculated VCD spectra of the reaction product 3a.

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes.

Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin-spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five-membered rings of 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes was studied. Experimental (3)J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical (3)J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended.

Stability and conformation of polycopper-thiolate clusters studied by density functional approach.

Quantum chemical studies of biologically relevant copper thiolate clusters can afford unique information about energetic principles of their formation and structure, which is important for understanding the basic principles of their formation and functioning in biological systems. In the current study, we used quantum chemical methods for the investigation of the structure and stability of Cu(x)S(y)-type clusters that serve as models for different copper thiolate clusters or for their intermediates in a variety of copper proteins. Density functional theory based modeling was performed including solvent effects for water and protein-like environments. Thermodynamic parameters (DeltaH, DeltaS, DeltaG) were calculated in order to assess the effect of thermal contributions to the formation energies of various copper thiolate clusters. The all-tricoordinated polycopper thiolate cluster [Cu4(SMe)6]2- turned out to be the most stable structure among the calculated ones. This result is in agreement with the prevalence of this type of clusters in various copper proteins with no sequence homology that contain six cysteine residues. The cooperativity of formation of [Cu4(SMe)6]2- can be inferred from the significant energy differences between intermediary clusters. Among tetrathiolate structures, [Cu2(SMe)4]2- was the most stable one. This cluster is also found in many copper proteins. Influence of slight structural perturbations on the energetics of copper thiolate clusters is also analyzed and discussed.

Enantioselective organocatalytic Michael addition of aldehydes to beta-nitrostyrenes.

The utility of C(2)-symmetric bipiperidine and bimorpholine derivatives as organocatalysts in the Michael addition of enamine intermediates formed from aldehydes to nitroolefins has been demonstrated. The best results were obtained when the reaction was run in the presence of (2R,2'R)-N-iPr-bipiperidine. The products were formed via an enamine intermediate with high diastereo- and enantioselectivity with relatively short reaction times.

Possible high-pressure structures of sulfur trioxide.

Calculations with the linearized augmented plane wave method indicate that several high-density forms of sulfur trioxide should be accessible at pressures above 29 GPa, with densities up to 1.7 times larger than the presently known forms of solid SO3.